Abstract: A new sequence analysis of peptides by using nano-electrospray ionization fourier transform ion cyclotron resonance tandem mass spectrometry was demonstrated in the study, and the angiotensin II was chosen as an example. Combined with the collision induced dissociation, FT-MS provides precise mass/charge ratio, but also structure information of the selected peptide. The fragment ions were identified on the basis of the precise molecule weight and peptide sequence was determined successfully. Nano-ESI FT-ICR MSn was determined as useful tool for identification of the unknown peptides, especially for the trace peptides isolated from gel electrophoresis.